Drugs and Breast Milk
The Department of Clinical Pharmacology at Otago University’s Christchurch School of Medicine and Health Sciences, and
Christchurch Hospital has released its latest research into drugs and breast milk. The Department has a long-standing
interest in the impact of drugs on babies ingesting breast milk from mothers on medication, and this further study
extends this investigation.
The focus of the latest research is the drug metformin, often used in the management of type 2 diabetes, and polycystic
ovary syndrome. There has been concern about the possible transfer of the drug to breastfeeding babies if their mother
is on this drug, and to what extent the baby absorbs metformin through breast milk.
The study has been carried out in association with the Canterbury DHB Diabetes Centre and the School of Pharmacy,
University of Otago, and has been submitted by Sharon Gardiner for her Masters of Clinical Pharmacy degree.
“What this study reveals is that although the baby is exposed to some metformin, the amount is well below the level
where clinicians and mothers need to be concerned,” says Professor Evan Begg, head of the Clinical Pharmacology
Department. “Our tests with mothers and babies have revealed for the first time that babies ingested metformin at just
0.2% of the maternal dose, after correction for body weight. This is well below the 10% safety threshold.”
The method used to determine exposure to a drug like metformin is by measuring its level in the blood of the mother and
baby, and also in breast milk. Although metformin was found in breast milk, no adverse effects were observed in the
three babies who were exposed to metformin in milk, and the drug could not be detected in the blood of the two babies
who were tested.
One of the other interesting aspects is that modelling indicated there should be much higher concentrations of metformin
in breast milk than was actually the case. This suggests there may be some unusual mechanism that keeps metformin out of
milk. This is now being investigated.
This result adds to other research into drugs and breast milk in the Department. Studies of other drugs, such as
quinapril, paroxetine and olanzapine, have indicated the transfer of these medications to babies through breast feeding
is below the clinical safety levels. For some other drugs such as fluoxetine (Prozac), drug transfer may be above the
usual safety limits, and breastfeeding is not recommended.